This application claims the priority of Application No. 2000-22218, filed Jan. 31, 2000 in Japan, the subject matter of which is incorporated herein by reference.
The present invention relates to a contactless (noncontact) interface circuit for contactless data transmission and a method for clamping a supply voltage; and more particularly to, a contactless (noncontact) interface circuit that controls a supply voltage for application to an internal circuit so that the supply voltage does not exceed a breakdown voltage.
In recent years, contactless (noncontact) IC cards have been developed and put to use. A conventional IC card includes, as shown in FIG. 1, a receiving/rectifying circuit, which receives voltage (alternating voltage) supplied from a coil in a coupler and rectifies the received voltage. The conventional IC card further includes a high supply voltage clamp circuit, which controls a power-supply voltage generated by the receiving/rectifying circuit so that the supply voltage does not exceed the breakdown voltage of circuit elements in the IC card. The high supply voltage clamp circuit includes diode-connected N type MOS transistors N1 to Nn and a controlling NMOS transistor, which turns on when a( predetermined voltage is applied to a node, and starts a clamping operation. Such a predetermined voltage is applied to the node, when the supply voltage becomes higher than the threshold voltages (breakdown voltages) of the N type MOS transistors N1 to Nn.
The threshold voltage of the N type MOS transistors N1 to Nn are uneven due to structure unevenness thereof. As a result, when the threshold voltage of the N type MOS transistors N1 to Nn shifts too low a value the clamping operation starts even if the a supply voltage is not yet high enough, so that undesirable electric current flows therethrough. Therefore, the transmission distance of electric power between the coil in the coupler and the coil in the IC card is shortened. On the other hand, when the threshold voltage of the N type MOS transistors N1 to Nn shift to too high a value, the clamping operation does not start even if the supply voltage becomes higher than the breakdown voltage of the inner circuit elements of the IC, so that an over voltage is applied to the circuit elements in the IC card. As a result, the circuit elements in the IC card may be damaged.
It is possible to reject defective IC cards in which N type MOS transistors N1 to Nn have uneven threshold voltages, however, the yield rate is decreased because many IC cards may be rejected.
Accordingly, an object of the present invention is to provide a contactless (noncontact) interface circuit that starts a clamping operation of a supply voltage accurately at an appropriate timing.
Another object of the present invention is to provide a method for clamping a supply voltage, in which a clamping operation starts accurately at an appropriate timing.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to a first aspect of the present invention, a contactless interface circuit includes a supply voltage generating circuit (10) which generates a supply voltage (V1);
a reference voltage generating circuit (20) which generates a predetermined reference voltage (V2);
a resistance circuit (22), provided with a dividing point (A) at which the supply voltage (V1) is divided, the voltage at the dividing point (A) being identical to the reference voltage (V2) when the supply voltage (V1) is identical to a predetermined clamp voltage (V0);
a decision circuit (A1) which detects whether the voltage at the dividing point (A) in the resistance circuit (22) reaches the reference voltage (V2); and
a control circuit (H10) which controls the supply voltage in response to an output signal of the decision circuit (A1) so that the supply voltage (V1) does not exceed a predetermined breakdown voltage.
In the resistance circuit a resistance value is determined so that the dividing point voltage corresponds to the reference voltage (V2), when the supply voltage (V1) is detected to be identical to the clamp voltage (V0). The decision circuit (A1) detects whether or not the dividing point voltage increases and reaches the reference voltage (V2).
The control circuit (H10) controls the supply voltage (V1) in response to the output of the decision circuit (A1) so that the supply voltage (V1) becomes lower than a predetermined breakdown voltage.
According to a second aspect of the present invention, a method for clamping a supply voltage (V1) for contactless interface includes the steps of: providing a clamp voltage (V0); providing a reference voltage (V2); dividing the supply voltage (V1) to provide a divided supply voltage; comparing the divided supply voltage with the reference voltage (V2); and controlling the supply voltage (V1) based on the result of the comparison so that the supply voltage (V1) does not exceed a predetermined breakdown voltage.
According to a third aspect of the present invention, a contactless IC card is provided with the contactless interface circuit, according to the first aspect of the present invention.